1. Field of the Invention
The invention relates to a heat-sensitive recording material comprising a web-shaped substrate and at least one heat-sensitive recording layer on either or both of the sides of the web-shaped substrate, and provides that this heat-sensitive recording layer contain a novel type of color acceptor combination.
2. Description of the Prior Art
Recording materials that are heat sensitive and react color-formingly to the external supply of heat have been known for many years and enjoy a basically undiminished popularity, inter alia because their use is associated with immense advantages for businesses issuing tickets and/or sales receipts. Since the color-forming components, i.e., the dye precursors as well as the color acceptors that react therewith on heating, also known as (color) developers, reside in the recording material itself in such a heat-sensitive recording process, the thermal printers, which are free from toner and ink cartridges and which do not require regular servicing, can be installed in large numbers. This innovative technology has thus become pervasive in the retail trade and in public transport in particular.
In the recent past, however, increasing concerns have been raised about the environmental compatibility of certain color acceptors, and these, while not necessarily being science based, cannot simply be disregarded by the industry and particularly by commerce. Particularly heat-sensitive recording materials containing                bisphenol-A, i.e., 2,2-bis(4-hydroxyphenyl)propane, and        bisphenol-S, i.e., 4,4′-dihydroxydiphenyl sulfone, have attracted criticism, and this led primarily to an increased use of        Pergafast® 201, i.e., N-(p-toluenesulfonyl)-N′-3-(p-toluenesulfonyloxyphenyl)urea, from BASF Corporation, and        D8, i.e., 4-[(4-(1-methylethoxy)phenyl)sulfonyl]phenol.        
EP 2 574 645 A1 proposes a heat-sensitive recording material comprising a substrate supporting a heat-sensitive recording layer comprising a dye precursor and a color acceptor. The color acceptor contains a molecule from an n-basic acid and oligomers of polylactic acid which are grouped n-fold around said acid or is formed by the molecule. The disclosure of this document similarly reveals that a color acceptor as described above is optionally combinable with other color acceptors selected from the list comprising:                bisphenol-A, i.e., 2,2-bis(4-hydroxyphenyl)propane,        bisphenol-S, i.e., 4,4′-dihydroxydiphenylsulfone,        Pergafast® 201, i.e., N-(p-toluenesulfonyl)-N′-3-(p-toluenesulfonyloxyphenyl)urea, from BASF Corporation, and        D8, i.e., 4-[(4-(1-methylethoxy)phenypsulfonyl]phenol,except that such a possible combination will not be very important in commercial practice with regard to bisphenol-A and bisphenol-S, since the marketplace is interested in heat-sensitive recording materials where the color acceptors are environmentally friendly to a particular degree. The otherwise highly informative document does not reveal any prompt whatsoever regarding the combination of color acceptors based on oligomers of polylactic acid with further color acceptors beyond the components listed above.        
US 2005 282,704 A1 discloses the possible use of copolymers of polylactic acid as energy absorbers in heat-sensitive recording materials. German Laid-Open Specification DE 22 53 772 A1 proposes a recording sheet having a color developer layer, which in addition to an acidic polymer, contains a metal salt or an organic acid of carbon, for example an aliphatic carboxylic acid, specifically inter alia 2-hydroxy-2,4-dimethylpentanoic acid.
US 2004/258,857 A1 proposes a nonreactive recording material whose elastic substrate comprises polylactic acid. US 2005/112,302 A1 in turn discloses an inkjet recording element the ink-permeable coating of which comprises a material based on polylactic acid.